1. Field of the Invention
The present invention relates to electrophotographic color image forming apparatuses that employ a rotary-method development process.
2. Description of the Related Art
In a full-color image forming apparatus of the related art that includes a single image bearing member and a rotatable support member integrally supporting a plurality of developing devices, the image forming apparatus employs a development process in which the developing devices are sequentially switched at a predetermined timing so as to develop an electrostatic latent image formed on the surface of the image bearing member.
An image forming apparatus that develops an electrostatic latent image formed on the surface of a single image bearing member by using a rotatable developing-device support member (i.e., a rotary) that integrally supports a plurality of developing devices in this manner to sequentially switch the developing devices is called a rotary-type image forming apparatus.
Japanese Patent Laid-Open Nos. 2005-148319 and 11-15265 disclose examples of such a rotary-type image forming apparatus of the related art.
Generally, in a rotary-method development process, it is necessary to sequentially bring toner bearing members, included in the developing devices provided in correspondence to toners of respective colors, into and out of contact with the surface of the image bearing member at a development position. The switching of the developing devices is performed by rotating the developing-device support member while the developing devices are set out of contact with the surface of the image bearing member.
In the image forming apparatus of the related art, the developing-device support member is moved in the radial direction (i.e., towards the center of rotation) of the image bearing member by using, for example, a cam having a driving force so as to bring the toner bearing member into and out of contact with the surface of the image bearing member.
As a drive transmission method of transmitting a driving force to the toner bearing member provided in each developing device, Japanese Patent Laid-Open No. 11-15265 discusses a method of transmitting a rotational driving force to the toner bearing member by joining a coupling in the developing device to a coupling in the main apparatus body.
In this case, the coupling in the main apparatus body is in a retreated state to prevent it from interfering with the rotation of the developing-device support member, and when the developing-device support member is rotated to a predetermined position from that state, the developing-device support member moves toward the coupling in the developing device so as to engage with the coupling in the developing device.
Therefore, a rotary-type image forming apparatus requires a time period for allowing the coupling in the developing device to engage with the coupling in the main apparatus body so that the toner bearing member can be rotated.
However, the rotary-type image forming apparatus of the related art has the following problems.
In the image forming apparatus of the related art, since the developing-device support member is configured to be moved in the radial direction of the image bearing member to bring the toner bearing member into and out of contact with the surface of the image bearing member, a space for moving the developing-device support member is necessary.
In addition, a cam, for example, is required as a driver for moving the developing-device support member in the radial direction of the image bearing member for performing the operation for moving the toner bearing member into and out of contact with the surface of the image bearing member.
In other words, the rotary-type image forming apparatus of the related art requires a driver as well as a space for entirely moving the developing-device support member in the radial direction of the image bearing member to bring the toner bearing member into or out of contact with the surface of the image bearing member. Therefore, it is difficult to achieve size reduction of the main apparatus body, as well as cost reduction.
As a countermeasure against such problems, the operation for moving the toner bearing member into and out of contact with the surface of the image bearing member may conceivably be performed by, for example, directly utilizing the rotation of the developing-device support member instead of using a driver, such as a cam.
However, if the operation for moving the toner bearing member into and out of contact with the image bearing member is to be performed by utilizing the rotation of the developing-device support member, it becomes difficult to ensure the time for engaging the coupling in the developing device with the coupling in the main apparatus body.
As a result, the toner bearing member comes into contact with the surface of the image bearing member before the two couplings engage with each other and the toner bearing member is rotationally driven. If the toner bearing member in a non-rotating state comes into contact with the surface of the image bearing member, the toner on the toner bearing member becomes scraped off by the image bearing member, creating a fog on the surface of the image bearing member and thus leading to an image defect.